FOOD AND AIR TURNED INTO FLESH AND ENERGY 97 



the cells — carbon dioxide and other waste matters — diffuse 

 into the liquid and from this to the blood through the 

 capillary walls. In fact each tissue-cell feeds, like cer- 

 tain one-celled animals, by absorption from a liquid 



Fig. 62. 



Fig. 63. 



Fig. 62. — Diagram of the circulation of the blood in a mammal; a, auricles; 

 /, lung; Iv, liver; p, portal vein bringing blood from the intestine; 7', 

 ventricles; the arrows show the direction of the current; the shaded 

 vessels carry venous blood, the others arterial blood. (From Kingsley.) 



Fig. 63. — Heart of cat, dorsal view; a, right ventricle; b^ left auricle; t", 

 right auricle; d, vena cava inferior; e^ vena cava superior; /", aorta. 

 (After Reighard and Jennings.) 



medium, but by means of the circulation this liquid has a 

 prepared food constantly brought to it. 



We may ask how the blood carries the oxygen. In 

 the vertebrates part of the blood consists of little bodies 

 called the red corpuscles. The color of these is due to 

 a chemical substance called hEemoglobin. This has the 



